1. Field of the Invention
The present invention relates to a methacrylic resin cast plate having a high transparency and an excellent impact resistance, and a process for the preparation thereof.
Methacrylic resin cast plates have an excellent transparency and surface gloss and a good weatherability and mechanical properties, and are widely used as illuminators, signboards, various construction materials and the like. Nevertheless, these cast plates do not have a completely satisfactory impact strength and are regarded as being brittle. The present invention is concerned with an improvement of the impact strength of the conventional methacrylic resin cast plates.
2. Description of the Related Art
As the means for improving the impact resistance of a methacrylic resin cast plate, there has been proposed a process in which a rubber component is dissolved in methyl methacrylate or a monomer mixture composed mainly of methyl methacrylate and the cast polymerization thereof is carried out by using an appropriate initiator.
For example, there have been proposed a process in which an olefin/acrylic acid ester rubbery copolymer is dissolved in monomeric methyl methacrylate and the mixture is polymerized in a cell constructed by two glass sheets, as disclosed in Japanese Examined Patent Publication No. 52-1759, and a process in which a solution of a vinyl acetate/ethylene copolymer in monomeric methyl methacrylate is partially polymerized, the phase conversion is allowed to occur to prepare a vinyl acetate/ethylene copolymer-containing partial polymerization product and the cast polymerization thereof is carried out, as disclosed in Japanese Unexamined Patent Publication No. 60-144308.
In the process disclosed in Japanese Examined Patent Publication No. 52-1759, the compatibility between the methacrylic resin and the rubber component is poor and a satisfactory impact resistance cannot be obtained. In the process disclosed in Japanese Unexamined Patent Publication No. 60-144308, the amount of the rubber component that can be dispersed in the methacrylate resin is limited, and therefore, the obtainable impact resistance is limited and a high impact strength cannot be realized.
As the means for improving the compatibility between methyl methacrylate and a rubber component, Japanese Examined Patent Publication No. 47-16186 proposes a process in which a monomer such as methyl methacrylate is graft-polymerized onto a vinyl acetate/ethylene copolymer, the obtained graft copolymer is dissolved in monomeric methyl methacrylate, and the obtained solution is subjected to cast polymerization. According to this process, the impact resistance is improved, but the excellent transparency and surface gloss inherent to cast plates become poor.
To obtain an enhanced impact resistance, a rubber component must be dispersed in a methacrylic resin with an appropriate particle diameter. In all of the foregoing processes, since a copolymer, which is not particulate, is used as the rubber source, the particle diameter of the rubber component, which has been rendered particulate by the cast polymerization, must be controlled according to the polymerization conditions and the like. Accordingly, the polymerization conditions must be strictly controlled and the physical properties other than the impact resistance, for example, the transparency and surface gloss of the obtained plate are poor.